Field of the Invention
This invention relates to a system for generating a reliable neutron count rate for the random emission of neutrons in a subcritical nuclear reactor. The invention incorporates a method and apparatus for the elimination of noise from such a pulse signal.
Background of the Invention
Neutron emission in a subcritical nuclear reactor is monitored to determine the power level, to assure that an event is not taking place which would result in the reactor unintentionally going critical, and to regulate reactivity during start-up. It is important that a reliable measure of neutron emission be available to perform these functions.
The task of generating such a reliable measure of neutron emission is made difficult by the fact that the rate of neutron emission in a subcritical reactor is so low that it is a random event. That is, there are frequent bursts of neutrons occurring between periods of little or no activity. It is known to utilize Poisson statistics to predict the neutron count rate in nuclear reactors for design and analysis purposes.
The sensors used in what is known as the source range instrumentation to detect neutron emissions in a subcritical reactor generate a pulse in response to each emission. These pulses are counted over recurring periods of time to generate a neutron pulse count rate. Unfortunately, the amplitude of the pulses generated by these sensors is not very high compared to the considerable noise that can be present. There are many sources of such noise. Since the signal level is so low, electronic noise from circuit components in the measuring system can be a factor. Also, gamma rays can trigger pulses in some sensors. The electronic noise and gamma ray noise tend to be lower in amplitude than the pulses produced in the sensor by neutrons. Other types of noise induced by outside events such as the operation of motors or other electrical equipment can result in large amplitude spikes in the neutron detection channels.
Noise has two kinds of effects on the signal generated in the neutron detector circuits. One effect is frequency bursts which can result in erroneous high count rates which can produce a spurious alarm and/or an unnecessary response. The other effect is to mask or obliterate the true count rate, resulting in an erroneous low count rate reading.
U.S. Pat. No. 4,582,672 discloses a system for detection of conditions which could lead to inadvertent critically in a nuclear reactor. In this system, the pulse signal from a neutron detector is converted to a logarithmic analog signal which is then pulse modulated for input into a digital computer. The computer continually generates average neutron count rate signals for incremental time periods and stores them for a preselected time period. At the end of each incremental time period, the latest average count rate signal is compared with the oldest, and preferably each of the intervening stored values, and if it exceeds any of them by a preselected multiplication factor, an alarm is generated. Specifically, the system is adapted to detect a boron dilution event.
While this system, to some extent, reduces the effect of frequency burst noise, further improvement is desirable. The system of U.S. Pat. No. 4,582,672 also does not deal with noise spikes which could mask the true neutron count rate.
There is a need therefore for an improved system for determining the neutron count rate in a subcritical nuclear reactor.
There is a more particular need for such a system which better filters out noise from the pulse signal.
There is a further need for such a system which filters both noise bursts and noise spikes from the pulse signal.
There is additional need for such a system which provides an indication of the quality of the pulse signal being generated.
There is also a need for such a system which limits deviations in the pulse count rate based upon the recent history of the pulse count rate signal.
There is a related need for such a system which will, however, allow the pulse count rate to track with reasonable accuracy true changes in the pulse count rate.